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The expansion of three solid solutions of the ferrite phase, C6A2F, C4AF and C6AF2, in the presence of CaSO4 · 2H2O, Ca(OH)2 and siliceous powder with water/solid ratio of 0.3, has been compared with that of two reference mortars: the first containing C3A, Ca(OH)2 and CaSO4 · 2H2O, the second C3A doped by 4,5% Fe2O3, Ca(OH)2 and CaSO4 · 2H2O. The expansion of mortars, based on the ferrite phase, has been observed to be a function of Al2O3 contents, and is much weaker than that of mortars based on C3A. For these last ones, the expansion decreases from C3A to doped C3A. The expansion due to the reaction of monosulfoaluminate and gypsum is much quicker if yielded by C3A. On the contrary that one due to the monosulfoaluminate yielded by ferrite is very small. Owing to these results, it seems that the low ettringite contents and the lower expansion of ferrites compared with the C3A are due not only to their low reactivity, but also to the formation of amorphous ferric compounds that cause a lowering of the hydration.